It is known that present stored-program control systems have memories organized in a hierarchic structure, including fast-access memories for on-line programs and data (main memories) followed by other memories, generally with slower access, for programs and data of less immediate and frequent use (mass memories). The latter memories often act also as auxiliary stores for the main memories, that is they contain also semi-permanent data and on-line programs necessary to allow the control system to resume its normal operation when a failure occurs in the main memories.
Till now, mass memories consisted usually of disk units, magnetic tapes or drums because, owing to the state of the art, these devices alone combined large storage capacity with low cost.
However, magnetic memories present some inconveniences, namely:
they cannot attain sufficiently high operating speed, chiefly fast access time; PA0 they cannot ensure a sufficiently high "system availability" (meaning probability of finding the system operating at any moment), owing to the frequent interventions necessary to maintain the efficiency of the units; this is due to the fact that the magnetic units have moving mechanical parts that require an initial running-in and present wear phenomena that can also require preventive maintanance.
For these reasons, studies aimed at obtaining memories of different types mainly for small and medium capacity (for instance up to 10 million words) have become very important; owing to the development of techniques used to build solid-state components, these studies have been directed toward highly integrated components and more particularly toward charge-coupled devices.
A memory of this type with operating characteristics very similar to those of a disk unit is already commercially available.
Such a solid-state memory intrinsically has high operating speed as well as good reliability and easy-maintenance characteristics; moreover, it exhibits good modularity enabling an initial use of rather small units that can thereafter be supplemented according to requirements.
Still, this memory presents certain drawbacks that limit its utility in telecommunication-system control; thus, it has no facility for automatic error correction and is organized by "bytes", that is by 8-bit words.
Since in telecommunication applications the control system must be in service continuously, it is important for the mass memory to be provided with self-correcting means preventing the system from becoming disabled during the time necessary for detecting the cause of the error and remedying same; self-correction provides an efficient protection of the stored data so that they do not get lost and can be used by a possible auxiliary unit put into service by a reconfiguration system.
In fact, processing systems with severe reliability requirements need usually a plurality of mass-memory units. On the other hand, if redundant parts for replacing any malfunctioning unit are provided within one of those memory units, the reliability requirements of the processing system could be met by a single mass-memory unit, affording significant savings.
Moreover, for both speed and flexibility purposes in the telecommunication field and in processing generally, the control system must operate on words of 16 bits or more.
To achieve flexibility on the actual length of the words, on the number of redundancy bits necessary for self-correction and on the number of spare parts, a memory should be organized by bits and should include many modules each storing one bit of a plurality of words.